1. Field of the Invention (Technical Field)
The present invention relates to wavelength modulation spectroscopy.
2. Background Art
Dual-modulation line-locking, described in D. Bomse, Applied Optics 30:2922-4 (1991) and in the parent applications to the present application, listed above, is a useful application of optical spectroscopy for continuous monitoring of a selected species. The technique yields high sensitivity measurements of an optical absorbance generated by the target species while also providing simultaneous wavelength stabilization of a spectroscopic light source. In particular, dual-modulation line-locking improves wavelength modulation spectroscopy, which is a spectroscopic method that is easily implemented with wavelength tunable light sources such as diode lasers, by removing baseline fluctuations that add error to the absorbance measurements.
In Bomse's original dual-modulation line-locking method, the wavelength of a light source is modulated at two frequencies. Typically, a beam splitter diverts part of the light through a sample region and onto a detector such as a photodiode; the remainder of the light passes through a reference cell filled with an amount of the target species and then impinges on another detector. Optical absorption by the species in the sample and reference paths converts some of the wavelength modulation into synchronous amplitude modulations of the light, with the magnitude of the induced amplitude modulations being proportional to the size of the absorbances. The magnitude of the absorbance can be directly related to the absolute concentration of the absorbing species. Output from the sample leg detector is processed by two sequential demodulations to provide a signal that is proportional to the sample leg optical absorbance, hence proportional to the amount of target species within the path. One advantage of the sequential demodulation is that the absorbance measurement is free of baseline fluctuations. Output from the reference leg detector is also processed by two sequential demodulations, but the frequencies of the reference waveforms used for the demodulations are selected such that the demodulated spectral waveform can be used as a feedback signal for wavelength stabilization of the light source. Ideally, the reference leg signal is zero when the light source wavelength is coincident with the center wavelength of the absorption feature, and the signal varies linearly with small displacements from the center wavelength. This use of an optical absorbance for wavelength stabilization is generally known as line-locking and can be implemented using wavelength modulation spectroscopy as well as frequency modulation spectroscopy.
Two problems may arise, however, when implementing line-locking. The first occurs because the line-locking feedback signal can be approximately zero when the spectroscopic source wavelength is coincident with the center of the reference absorption feature or when the source wavelength is far removed from line center. The conventional line-locking signal cannot be used to distinguish between these two cases. The other problem arises when the amount of absorber material in the reference path changes. This introduces uncertainty into the amount of wavelength correction that needs to be applied in response to a given feedback signal. In other words, changing the amount of reference absorber material varies the gain of the feedback loop and can cause instabilities in the line-locking.
FIG. 3 is a schematic diagram of an undisclosed method (developed by the present inventors) that uses a second demodulator to provide a signal proportional to the optical absorbance within the reference leg optical path. The current applied to a diode laser 126' such as a Lasertron model QLM5S890-002 is modulated sinusoidally at two frequencies simultaneously, .OMEGA.=1 MHz and .omega.=10 kHz. It is well known that changing the current applied to a diode laser is a simple method for changing the wavelength of the laser light. The 1 MHz waveform is obtained by applying the output from a 3 MHz TTL oscillator 118' such as a Dale model X0-43B to a down counter 122' such as a Texas Instruments 74HC160N integrated circuit configured as a divide-by-three circuit component. Output from the counter is bandpass filtered 132' using a device similar to a TTE, Inc. series KC4-1M bandpass filter to produce a 1 MHz sine wave that is used as the first modulation frequency. The 10 kHz modulation waveform is supplied by a function generator 120' such as a Stanford Research Systems model DS345 function generator. The DC portion of the laser current is supplied by a current source such as an ILX model 3722b diode laser controller.
A beamsplitter 128' divides the laser beam into a sample leg portion 130' used for determining the absorbance of an unknown amount of the target species and a reference leg portion used for line-locking. The latter beam passes through a reference cell 138' filled with some of the target species and onto a photodiode detector 140' such as an Epitaxx ETX1000T InGaAs photodiode. The signal produced by the photodiode is demodulated using demodulator #1 142' such as a Mini-Circuits SRA-6 mixer; the local oscillator for the demodulator is supplied by the 3 MHz oscillator 118'.
Output from demodulator #1 is applied to a lowpass filter 144'. The lowpass filter is a device similar to a TTE, Inc. LC7-series filter and removes AC signal components at 1 MHz and 10 kHz (frequencies .OMEGA. and .omega.) and their harmonics. Output from the lowpass filter provides the unnormalized error signal that is used for laser wavelength stabilization.
Output from demodulator #1 is also applied to demodulator #2 150 which uses frequency .omega. from source 120' as its local oscillator. Demodulator#2 150 is a device similar to Analog Devices AD630 integrated circuit. It produces a voltage proportional to the amplitude of the 10 kHz AC signal that is present on the output of demodulator #1 142'. When the laser wavelength is coincident with the center of the optical absorbance and the phase of the local oscillator is properly adjusted, the output of demodulator #2 150 is directly proportional to the optical absorbance in the reference path. Dividing the error signal obtained at the lowpass filter by the absorbance signal yields a normalized error signal that provides improved reliability for laser wavelength stabilization.
The present invention improves on dual-modulation line-locking by processing the line-locking signal to 1) verify that the light source wavelength is coincident with the absorption line center, and 2) maintain constant gain in the wavelength stabilization feedback loop.